In sheep, bred following hemihysterectomy, and in sheep, in whom a partial ablation of placental sites is performed during pregnancy, oxygen consumption of the fetus and placenta will be measured in situ in order to determine the role of placental surface area as a limiting factor in fetal oxygenation. The role of diamox in trapping carbon dioxide on the fetal side of the placenta will be studied as a model for producing unilateral fetal respiratory acidosis and studying its effect on umbilical blood flow and fetal oxygen consumption. The Panigel model will be used to study the usefulness of blood substitutes in studies of placental function, particularly with respect to the true oxygen and carbon dioxide pressure gradient and the action of diamox.